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need more compression

  1. #1
    Senior Member whiskeybent hondo's Avatar
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    Default need more compression

    I bought a 13:1 468 gen IV big block Chevy that was built ten years ago and put in storage. The block has o-rings and the new dart heads are machined around the combustion chamber as well. My goal is to get more compression.


    The motor has SCE copper gaskets P/N 013524 which is a 4.520 bore, .043 thickness and a compressed volume of 11.307cc.


    The only reason I can think of running a gasket that big on a 4.310 bore motor would be to clear the intake valve where the relief is in the cylinder? The heads are pro 1 325’s and have a 2.25 intake valve. I put clay on the side of the cylinder into the block relief and spun the valve train, but it’s still difficult to tell how close the valve is to the relief.


    My plan is to order SCE copper gasket P/N 013322 which is a 4.320 bore, .021 thickness, and a compressed volume of 5.044cc.


    1.) Do you think the gasket will interfere with the intake valve?
    2.) Is there any other gasket that might work better considering the o-ringed block and heads?


    P.S. There is plenty of valve to piston clearance. Also this motor is not going in a boat.


    Thank you for the advice.

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  3. #2
    gn7
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    Quote Originally Posted by whiskeybent hondo View Post
    I bought a 13:1 468 gen IV big block Chevy that was built ten years ago and put in storage. The block has o-rings and the new dart heads are machined around the combustion chamber as well. My goal is to get more compression.


    The motor has SCE copper gaskets P/N 013524 which is a 4.520 bore, .043 thickness and a compressed volume of 11.307cc.


    The only reason I can think of running a gasket that big on a 4.310 bore motor would be to clear the intake valve where the relief is in the cylinder? The heads are pro 1 325’s and have a 2.25 intake valve. I put clay on the side of the cylinder into the block relief and spun the valve train, but it’s still difficult to tell how close the valve is to the relief.


    My plan is to order SCE copper gasket P/N 013322 which is a 4.320 bore, .021 thickness, and a compressed volume of 5.044cc.


    1.) Do you think the gasket will interfere with the intake valve?
    2.) Is there any other gasket that might work better considering the o-ringed block and heads?


    P.S. There is plenty of valve to piston clearance. Also this motor is not going in a boat.


    Thank you for the advice.
    A 2.25 valve will not hit the cylinder on a 4.310 bore. The large gasket is not much to clear the notch as it is size the head's chamber.
    The Felpro 1027 has a 4.370 bore and is oblong to clear the notches, but if you lay it on a Dart, AFR or Canfield you will find it hangs into the chamber on the plug side of the chamber. Not a huge deal with a O ring copper gasket because the fire ring is your wire. On a Felpro or Cometic, the smaller bore gasket will put the fire ring inside the chamber where it can't seal to the block.

    Using a gasket smaller than the chamber is not a good habit to get into, regardless if its a copper o ring gasket, or a Felpro/Cometic.
    How much compression were hoping to gain? .10 with a .040 gasket?
    If you use a .021 gasket, I take you have verified that the piston is about .020 in the hole?



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    Last edited by gn7; 09-01-2013 at 01:11 AM.

  4. #3
    Senior Member whiskeybent hondo's Avatar
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    Quote Originally Posted by gn7 View Post
    A 2.25 valve will not hit the cylinder on a 4.310 bore. The large gasket is not much to clear the notch as it is size the head's chamber.
    The Felpro 1027 has a 4.370 bore and is oblong to clear the notches, but if you lay it on a Dart, AFR or Canfield you will find it hangs into the chamber on the plug side of the chamber. Not a huge deal with a O ring copper gasket because the fire ring is your wire. On a Felpro or Cometic, the smaller bore gasket will put the fire ring inside the chamber where it can't seal to the block.

    Using a gasket smaller than the chamber is not a good habit to get into, regardless if its a copper o ring gasket, or a Felpro/Cometic.
    How much compression were hoping to gain? .10 with a .040 gasket?
    If you use a .021 gasket, I take you have verified that the piston is about .020 in the hole?
    Good info. Initially when I bought the motor I thought I was getting a 13:1 motor, but the heads ended up being 121cc instead of 118cc and the gasket is big. My goal is to get it closer to 13:1. Changing from .043 to .021 gets me from 12.28 to 13:1, not including ring land distance in the calc. I need to call ross to find that info out. I already re-assembled the short block.


    I put the 4.520 copper gasket over the combustion chamber and noticed it does slightly hang over on the intake side. Looks like I’m stuck with that bore gasket then. SCE does make a .021 gasket with the 4.520 bore size….or they also make a 4.380 in a .021 thickness as well…. Which would you choose?


    The piston is .013 in the hole, but I might put it on alky.


    Thank you GN

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  6. #4
    gn7
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    Quote Originally Posted by whiskeybent hondo View Post
    Good info. Initially when I bought the motor I thought I was getting a 13:1 motor, but the heads ended up being 121cc instead of 118cc and the gasket is big. My goal is to get it closer to 13:1. Changing from .043 to .021 gets me from 12.28 to 13:1, not including ring land distance in the calc. I need to call ross to find that info out. I already re-assembled the short block.


    I put the 4.520 copper gasket over the combustion chamber and noticed it does slightly hang over on the intake side. Looks like I’m stuck with that bore gasket then. SCE does make a .021 gasket with the 4.520 bore size….or they also make a 4.380 in a .021 thickness as well…. Which would you choose?


    The piston is .013 in the hole, but I might put it on alky.


    Thank you GN
    .034??
    I've never put a BBC piston that close to the head before. The SBC with relatively tight piston clearances territory.



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